Method and apparatus for eliminating crosstalk in active matrix liquid crystal displays

Abstract: The elimination of crosstalk between data lines and pixel cells in a thin film transistor/liquid crystal display is accomplished by applying a precharge voltage level for a given data signal level which also provides an equivalent to a compensation voltage for a prior scan line to a given data line for a time period less than the standard scan line period of the display, and applying the data signal to the given data line for the remainder of the scan line period.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention is generally directed to a method and apparatus for eliminating cross-talk in liquid crystal display devices. More particularly, the present invention is related to a display device in which means for preventing cross-talk between data lines and pixels is provided. BACKGROUND ART As explained in United States Patent No. 4,873,516 to Castleberry, a proper understanding of the present invention can only be had by understanding the operation of a liquid crystal display device and the problems of parasitic capacitance inherent in the structure of these devices. In particular, a liquid crystal display device typically includes a pair of substrates fixed a specified distance apart. This distance is typically approximately 6 microns. A liquid crystal material is disposed between the substrates. The substrates are selected so that at least one of them is transparent. If back lighting is provided as a means for providing or enhancing the display and image, it is required that both substrates be substantially transparent. On one of these substrates there is disposed a transparent ground plane conductor typically comprising material such as indium tin oxide (ITO). The opposing substrate contains a rectangular array of individual electrode elements, called pixel electrodes. A semiconductor switch (preferably a thin film transistor) is associated with each of these pixel electrodes and is typically disposed on the substrate containing these electrodes. These transistor switches are usually based upon either amorphous silicon or polycrystalline silicon technology. At present, amorphous silicon technology is preferred because of its lower process temperature requirements. In effect, the aforementioned structure results in a rectangular array of capacitor-like circuit elements in which liquid crystal material acts as a dielectric. Application of voltage to a pixel electrode results in an electro-optical transformation of the liquid crystal material. This transformation is the basis for the display of text or graphical information seen on the device. It is noted that the invention herein is particularly applicable to the above-described display device in that each of the pixel electrodes is associated with its own semiconductor switch which may be turned on or off so that each individual pixel element may be controlled by signals supplied to its associated semiconductor switch. These semiconductor devices essentially act as electron valves for the deposition of charge on individual pixel electrodes. Each transistor is provided with a scan line signal and a data line signal. In general, there are M data lines and N scan lines. Typically, the gate of each transistor switch is connected to a scan line and the source or drain of the transistor switch is connected to a data line. In operation, a signal level is established on each of the M data lines. At this point, one of the N scan lines is activated so that the voltages appearing on the data lines is applied to the pixel electrodes through their respective semiconductor switch elements. A necessary consequence of the arrangement described is that each pixel electrode is surrounded on both sides by data lines. One of the data lines is the data line associated with the pixel electrode. However, the other data line is associated with an adjacent pixel electrode. This latter data line carries a different information signal. Also inherent in this structure are certain capacitive features. In particular, the pixel electrode and its opposing ground plane electrode portion form a capacitive structure. In addition, there are parasitic capacitances between each data line, and its surrounding pixel electrode elements. Moreover, there is a parasitic capacitance which exists between the source and drain of the semiconductor switch element. The parasitic capacitances permit undesired signals to be applied to the pixel electrodes. In a typical operational sequence, desired voltage levels are established on the data lines and a scan line is activated so as to apply these voltages to a single row of pixel electrodes. After a time sufficient for charging the liquid crystal capacitor, a different scan line is activated and a different set of data voltages is applied to a different pixel row. Typically, an adjacent pixel row is selected for writing video information. Thus, in a typical operation, one row of the display device can be written at one time, from the top to the bottom of the screen. In television applications, this top to bottom writing occurs in approximately 1/30th or 1/60th of a second. Thus, in this time period, a complete image is displayed on the screen. This image may include both text and graphical information. As is well known in the electrical arts, capacitive effects are generally proportional to area and inversely proportional to distance. Thus, in high resolution liquid crystal display devices, the parasitic capacitance effects are particularly undesirable because of the requirement for small spacing between the data lines and the pixel electrode. In typical applications contemplated herein, such as a television or computer display environment, the pixel electrodes are approximately 300 x 100 microns 2 and separated by a space of approximately 6 microns with an area of approximately 10 x 10 microns 2 being set aside from each pixel for the placement of its associated semiconductor switch element. Thus, it is found that in high resolution thin film transistor matrix addressed liquid crystal displays, the parasitic capacitance between the data lines and the pixel electrode is not insignificant when compared to the pixel capacitance. It is also noted that the parasitic capacitance between the data lines and the pixel electrode is increased by the presence of the parasitic source to drain capacitance in the switch element itself. In operation of such a display, the voltage on a pixel is set during its row address time. The semiconductor switch is then turned off and the voltage should remain fixed until the display is refreshed. However, any change in the voltage on an adjacent data line produces a change in the voltage on the pixel. In many drive schemes, the voltage on a data line typically varies between 0 and 5 volts, depending on how many elements in the column are turned on. This results in an uncertainty or cross-talk in the voltage on the pixel. In a design in which there are approximately 100 pixels per inch, this results in a maximum voltage error of approximately 0.2 volts RMS. While this is not critical for on-off displays, it is very significant for gray scale displays where changes in the voltage of 0.05 volts RMS are visible. One method for reducing, but not eliminating cross-talk of the kind discussed above is the use of a storage capacitor in parallel with C LC . This reduces the maximum error voltage. This method is commonly used at present but is undesirable, because it usually requires additional processing steps, because it can cause additional defects to be present and because it reduces the active area of the pixel elements. Another method for eliminating crosstalk is described in United States Patent No. 4,845,482 to Howard and Alt. Typical waveforms of this method are shown in Fig. 1(a) to Fig. 1(d). Fig. 1 (a), Fig. 1(b) and Fig. 1(c) are waveforms applied to successive gate lines while Fig. 1(d) is a typical data line signal. The elimination of crosstalk is accomplished by providing the data complement for each data when the gate line is inactive. It is clear that this method requires that a fraction of the line time (typically one half) be devoted to the compensation signals, with the transistors turned off. As a result, it demands a factor of two increase in switching speed which requires faster switching TFTs, more expensive drivers, and higher power consumption to drive the data lines. Conference Record of the 1988 International Display Research Conference, San Diego, US, Oct. 88, pp. 230-235 (W.E. Howard et al) discloses a crosstalk elimination technique in which each row of the display is addressed for a period T/2N rather T/N. The gate pulse width is reduced to T/2N, and during application of gate pulse, the data voltage Vi is applied to the column electrode as before. During the next time interval T/2N, the gate voltage is turned off and a data complement voltage (Vm-Vi) is applied to the data line, where Vm is a constant voltage. Essentially, this addressing sequence is one of data, data complement, data, data complement etc. with the gate pulse synchronized to the intervals of the data voltages. EP-A-0 288 discloses an active matrix liquid crystal display apparatus which employs compensation thin film transistors for compensating DC voltage level shifts that may cause afterimages. The thin film transistor compensates for a potential fluctuation occurring in the display electrode after the gate of the thin film transistor is selected. These transistors are operated by address and compensation pulses of a drive waveform on the scan bus line. SUMMARY OF THE INVENTION It is a principle object of the invention to provide a liquid crystal display and a method of operating the display wherein crosstalk is reduced or eliminated. It is a further object of the invention to provide a circuit for driving the pixels of a liquid crystal display which utilizes the method. It is another object of the invention to reduce crosstalk in a liquid crystal display without increasing the cost or power required to drive the pixels. These objects are achieved by the method according to claim 1 and a display according to claim 5. Effectively, in a liquid crystal display including a plurality of sequencially excited gate lines and a plurality of data lines the method for eliminating crosstalk between display elements comprises the step of exciting each data line for a time equal to a gate period so that changes in polarity of the data occurs during the first portion of the gate period (known as precharging). During a scan line time the first portion of the data signal has two purposes: (1) provide a compensation level for the previous data signal and (2) provide the precharge level for the upcoming data level. The second portion of the scan data signal provides the actual data volatge level. In accordance with a further embodiment of the invention, crosstalk is eliminated by starting the gate time at a change in polarity of the data signal and ending the gate time before the next successive change in polarity of the data signal. When polarity of the data signal is changed the display elements to receive the data are precharged. The precharge may include a compensation level of equal magnitude and opposite polarity to the previous data level. After precharging the data signal is changed to its intended level. In accordance with another embodiment of the invention crosstalk between display elements is eliminated by alternating the polarity of the data voltage supplied to the data lines for every adjacent row; precharging the display elements to compensate for previous data during a first portion of a line time; and charging the display elements to a final, intended value during at least a portion of the remainder of the line time. Also in accordance with another embodiment of the invention a display including a matrix of thin film transistor liquid crystal display cells driven by gate lines and data lines comprises gate signal means for applying a gate signal to successive ones of said gate lines for a gate signal period; and data signal means for applying to said data lines a data signal equal to a crosstalk compensation voltage minus data signal voltage for a previous gate signal period during a first portion of a current gate signal period, and for applying a voltage equal to a current data signal voltage for said current gate signal period to said data lines for a remainder of said current gate signal period. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1a to Fig. 1d represent timing diagrams for a prior art driving method. Fig. 2a to Fig. 2f represent timing diagrams for implementing the method according to the invention. Fig. 3 is a block diagram for a circuit for implementing the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 2(a), to Fig. 2(e), illustrate the waveforms applied to to successive gate lines. Fig. 2(f) illustrates the waveform applied to a data line. The polarity of the data voltage is alternated for every adjacent row, with N rows total. During the first half of the line time, the pixels are precharged to - V m + V i -1 (or V m - V i -1 ), which is the compensation level or for the previous data voltage, + V i -1 (or - V i -1 ). During the second half of the line time, they are charged to the final voltage + V i (or - V i ), which is the present data voltage. Thus, the entire line time is utilized to charge pixels. It is easy to calculate the RMS voltage at the liquid crystal resulting from the disclosed waveform, assuming a coupling factor α associated with the bypass capacitance; that is the capacitance that exists between the data line and the liquid crystal electrode. The expression for the RMS voltage at the ith row position is given by where we assume V i >0 and i =odd integer. The expressions for the other cases ( V i >0 and i =even integer; V i < 0 and i = odd integer; V i < 0 and i = even integer) give similar results. Also, the effects of the decay of the voltage have been neglected for simplicity. They are easily added, and do not change the conclusions. It can be seen by expanding this expression that there is cancellation of terms linear in α, which would normally be the dominant crosstalk terms. The expression then becomes The first term represents a small gain correction; the second term represents the second order crosstalk term proportional to α 2 . It is clear that the first order cross-talk term is eliminated. These expressions include only the terms describing the coupling from the data line to the liquid crystal electrode. There is also a coupling from the adjacent data line, but this can be included in a straightforward way, with the same cancellation. In this regard see the above mentioned U.S. Patent 4,845,482 to Howard and Alt, assigning a coupling coefficient β for the adjacent data line, then there are additional second order corrections proportional to β 2 and 2αβ. However, the first order terms linear in α and in β all cancel out. In general, the above result indicates that to achieve first order crosstalk elimination, V m can be set to any practical value. For the TFT/LCD case, where the TFT is operating in the linear region exhibits negligible drain to source voltage drop, V m can be set to zero. This scheme ( V m = 0) reduces the number of data driver voltage levels needed since the compensation voltage levels are equal to the data voltage levels. For other AM LCDs, such as MIM or diode configurations, there exits a bias drop from the data line across the switch to the liquid crystal capacitor. In these AM LCDs, V m should be chosen to eliminate directional dependent data voltage level charging, thus, avoiding a precharging level larger than the final data level. To achieves this, V m should be chosen such that V data (largest) - V m ≤ V data (smallest). Fig. 3 illustrates one analog addressing implementation, in accordance with the invention, for a multilevel grey scale matrix addressed pixel array 1. Serial data by row which for example could be provided from a frame buffer (not shown) is provided via data input line 2 to the first input of an analog toggle 4 and to the input of an inverter 6. The serial data on line 2 is provided twice so that the output of the toggle switch 4 is the serial signal A equal to D1,-D1,D2,-D2,D3,-D3, etc., where D1 represents the serial data V1 through VK at time t, where -D1 represents the serial data -V1 through VK at time t+T, D2 represents the serial data V1 through VK at time t+2T, etc. The crosstalk correction voltage level is provided, for example, as a bilevel signal, alternating from zero to -VM, via line 8 to the second input of an analog toggle 12 and to the input of an inverter 10. The output of analog toggle 12 is the serial signal B equal to 0, Vm, 0, -Vm, 0, Vm, etc. The correction voltage clock of analog toggle 12 and the serial data clock of analog toggle 4 are synchronized so that the serial data B from the output of analog toggle 12 changes when serial data A from the output of analog toggle 4 changes in such a manner, for example, so that serial data A and serial data B to the inputs of a summer 14 will be D1 and zero, followed by -D1 and VM, followed by -D2 and zero, followed by D2 and -VM, etc. The addition of serial data A and serial data B is accomplished by summer 14 in such a manner that the output Y will be the serial data D1, followed by (Vm-D1), followed by -D2, followed by (-Vm+D2), etc. A clock signal supplied on a data drive clock line 15 for a data driver shift register 16 will allow the data Y to be inputed in a serial fashion into the data driver shift register 16 at least K times faster then the parallel output 32, where K is equal to the number of data line outputs. A data driver reset line 18 and a data driver enable line 20 provide the syncronization between the Y serial data provided to shift register 16 and the parallel output on lines 32. The gate driver enable line 22, clock line 26 and gate driver reset line 28 provide the syncronization between gate driver 24 and data driver shift register 16 so that the bilevel signal output from gate driver 24 (one of the gate lines 30 from 1' to N) is syncronized to the parallel output from the data driver shift register 16. For every gate driver output signal duration, represented by T, the data driver shift register parallel output (from 1 to M) is composed of the crosstalk compensation signal during a first portion of T and then followed by the unadulterated data signal (no compensation) during the remaining portion of T, as shown in the waveform timing diagram of Fig. 2(f).

Claims: In a liquid crystal display including a plurality of sequentially excited gate lines and a plurality of data lines for exciting display elements of said liquid crystal display, a method for eliminating crosstalk between display elements comprising the steps of: exciting each data line with a data signal (V m -V i-1 ) including a compensation level for a first portion of a gate period equal in amplitude to the previous data signal level (V i-1 ) applied during the previous gate period and of a polarity opposite said previous data signal level; and applying a final voltage equal to the current data signal voltage (V i ) for the current gate signal period during a second portion of said gate period. The method of claim 1, wherein the data signal has a first compensation level of a first polarity and a second compensation level of a second polarity opposite said first polarity, said first compensation level and said second compensation level being applied in alternate gate periods. The method of claim 1 or 2 wherein said first portion of the gate period has a duration of substantially one half the gate period. The method of claim 1 to 3 comprising the further steps of alternating the polarity of the data voltage supplied to the data lines for every adjacent row. A display comprised of a matrix (34) of thin film transistor/liquid crystal display cells, each cell being defined by the intersection of one of a first plurality of data lines extending in a first direction and one of a second plurality of gate lines extending in a second direction which is at an angle to said first direction, with a given cell being turned on in response to the data line and the gate line that intersect at the cell having a data signal and a gating signal, respectively, applied thereto, comprising: gate signal means (24) for applying a gate signal to successive ones of said gate lines for a gate signal period; and data signal means (16) for applying to said data lines a data signal equal to a crosstalk compensative voltage (V m -V i-1 ) for a previous gate signal period including a compensation level equal in amplitude to the previous data signal level during a first portion of a current gate signal period, and for applying a final voltage equal to a current data signal voltage (V i ) for said current gate signal period to said data lines for a remainder of said current gate signal period. The display of claim 5, wherein said data signal means comprises: first inverting means for inverting the data signal to produce an inverted data signal; and first alternating means for applying one of the data signal and the inverted data signal alternately to said data lines. The display of claim 5 or 6, wherein said data signal means further comprises: a compensation signal source for supplying said compensation signal. The display of claim 5, 6 or 7, wherein said compensation signal source comprises: second inverting means for inverting said compensation signal to produce an inverted compensation signal; and second alternating means for applying one of the compensation signal and the inverted compensation signal to said data lines. The display of claim 5, 6, 7 or 8 further comprising summing means for summing an output of said first alternating means and an output of said second alternating means to produce said data signal applied to said data lines. The display of any one of claims 5 to 9, wherein said compensation signal includes a first compensation voltage of a first polarity and a second compensation voltage of a second polarity opposite that of said first polarity. The display of any one of claims 5 to 10, wherein said second compensation voltage and said first data voltage level are of equal absolute value. The display of any one of claims 5 to 11, wherein said second compensation voltage and said first data voltage level are of opposite polarity. The display of any one of claims 5 to 12, wherein said first portion of said current gate signal period has a duration of substantially one half of that of said current gate signal period.

Inventors: LIBSCH FRANK R, LIEN SHUI-CHIH ALAN, LIBSCH, FRANK R., LIEN, SHUI-CHIH ALAN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1994
Main classification: G09G3
IPC codes: G09G3, G02F1